1. Field of the Invention
The invention relates generally to a die casting apparatus and a die casting method, and more specifically to a technique of performing die casting with a cavity of a mold depressurized.
2. Description of Related Art
In die casting according to related art, the following technique is adopted: a prescribed amount of molten metal is fed by a pump from a molten metal holding furnace into a plunger sleeve having a molten metal feed port; and after the completion of feeding of the molten metal, a plunger tip is advanced by an actuator at a prescribed timing to inject the molten metal under high pressure from the plunger sleeve into a cavity of a mold (see, for example, Japanese Patent Application Publication No. 2013-208646 (JP 2013-208646 A)).
In a die casting apparatus according to the related art as described in JP 2013-208646 A, it is necessary to adjust the molten metal feeding voltage to be applied to the pump, based on the level of the molten metal in the molten metal holding furnace. Specifically, as the level of the molten metal is lowered, the molten metal pressure applied to the pump decreases, which makes it necessary to increase the molten metal feeding voltage to be applied to the pump. In view of this, as indicated in FIG. 6, in a die casting apparatus according to related art, the level of molten metal in a molten metal holding furnace is detected before feeding of the molten metal (step S01 in FIG. 6). Then, the molten metal feeding voltage to be applied to a pump is set for each injecting operation based on the detected level of the molten metal (step S02 in FIG. 6). Next, the molten metal is fed into a plunger sleeve (step S03 in FIG. 6). After that, the molten metal is injected under high pressure from the plunger sleeve into a cavity of a mold (step S04 in FIG. 6).
With this configuration, as illustrated in FIG. 7A, the molten metal is fed into the plunger sleeve by applying a prescribed molten metal feeding voltage to the pump during a period of molten metal feeding performed for each injection. However, when the amount of the molten metal in the molten metal holding furnace changes during the molten metal feeding after the level of the molten metal in the molten metal holding furnace is detected, the accuracy in the amount of the molten metal fed into the plunger sleeve may be reduced because adjustment of the molten metal feeding voltage applied to the pump based on such a change is not made. For example, when the amount of the molten metal in the molten metal holding furnace increases during the molten metal feeding, the molten metal pressure applied to the pump increases, which makes it necessary to lower the molten metal feeding voltage to be applied to the pump. In other words, when the molten metal feeding voltage maintained at the set voltage is applied to the pump, the amount of the molten metal fed into the plunger sleeve may increase. More specifically, when the level of the molten metal rises temporarily as indicated in a region α0 in FIG. 7B, the amount of the molten metal fed by the pump may increase temporarily as indicated in a region β0 in FIG. 7B.